Devices exist that supply a high-frequency low voltage to a compressor during shutdown during heating (for example, see Patent Literature 1).
Moreover, devices exist that supply a single-phase AC voltage having a higher frequency than that when a normal operation is performed to a compressor when it is detected that the temperature of the air conditioner's surroundings becomes low (for example, see Patent Literature 2).
Furthermore, devices exist that perform preheating by using copper loss of the wirings of the motor by performing the locked energization in which a DC current is caused to flow (for example, see Patent Literature 3).
Moreover, devices exist that enable heating at a constant electric power and with low noise without depending on the value of the voltage to be applied by driving the inverter with driving signals synchronized with a reference signal (for example, see Patent Literature 4).